The subject invention relates to a pre-processing apparatus for optimum overshoot control of an FM stereo system.
FM stereo radio broadcasting is a highly competitive medium. The need for loudness in FM has evolved as a result of this competitiveness and because FM is being used for broader markets such as country western, hard and soft rock, and other up-beat musical formats. Many FM stations are competing directly with AM stations and have developed a need for loudness.
An FM broadcaster must deal with modulation that is not only amplitude sensitive but frequency dependent as well. The maximum permissible deviation (100% modulation) is established by the FCC (Federal Communications Commission) as .+-.75 KHz. Further the FM transmitter includes a high frequency pre-emphasis network which has been standardized in the United States as one having time constant of 75 microseconds. This pre-emphasis is such that a greater deviation at the higher frequencies is produced so as to overcome the noise rejection characteristics of the FM system which decreases at the higher frequencies. An FM receiver for use in the United States includes a 75 microsecond de-emphasis network. The standard 75 microsecond pre-emphasis presents a frequency dependent limit imposed by the severe high frequency boost given the audio signal. To prevent overmodulation or "overshoot" (exceeding .+-.75 KHz) FM limiters or clipping circuits have been utilized. Two basic approaches to the elimination of overmodulation caused by the 75 microsecond pre-emphasis have been used. One approach is a selective attenuation of the high frequency program content based upon the amount of energy present in these higher frequencies. In other words, the bandwidth of the audio is dynamically rolled off in direct proportion to the high frequency signal level. As the high frequency energy content increases, high frequency roll off occurs and levels at 100% modulation level. The frequency response is the inverse of the 75 microsecond pre-emphasis curve. This is an effective way of controlling overmodulation, but is obviously at the detriment of the response quality. A second approach is clipping. Implementing this concept requires only pre-emphasizing the audio and hard clipping at all the peaks at 100% modulation. Harmonic distortion generated by this process is attenuated by the subsequent de-emphasis in the receiver and potential out-of-band radiation is suppressed by the low-pass audio filters. Although this second approach has met with a greater acceptance, overshoots still occur. One of the primary reasons for this overshoot, despite the FM limiter or clipper, is due to the low-pass in the stereo generators. All stereo generators employ low-pass filters to basically do the following: 1. prevent interference with the 19 KHz pilot; 2. limit aliasing distortion caused by high frequencies in the L+R channel leading into the upper or lower L-R sideband; 3. eliminate out-of-band spurious emission; and 4. reduce crosstalk in the stereo in SCA subcarrier. To accomplish these tasks, while maintaining a flat audio bandwidth to 15 KHz, rather sophisticated filters are employed. Some of these low-pass filters utilize transition rates in the area of 100 db per octave or more and are typically 40 to 50 db down at 19 KHz. Such filters most often exhibit a large non-linear phase shift which results in filter overshoots and ringing when clipped audio (approaching square wave excitation) is applied. These amplitude overshoots can be very severe and are caused by the non-linear phase shift allowing amplitudes of various frequencies to add in phase with resulting overshoot and by the so-called "Gibbs phenomenon". The non-linear phase shift can be corrected by an all pass filter that has an inverse phase of that in the stereo generator low-pass filter so that when combined in series, the resultant phase is linear. The "Gibbs phenomenon" effect is inherent in all filters and occurs when a complex wave (for example, a squared-off audio peak) is low-pass filtered and the harmonics lost. Some subtractive harmonics are lost (due to filtering), permitting larger amplitude peaks to be present after filtering than were present before filtering. By resorting to a complex non-linear filter where the resulting overshoots are subtracted from the waveform, a single filter stage may be designed to limit drastic amplitude overshoots. However, an FM exciter may include many band-limited stages or even other low-pass filters in the signal chain. In an effort to deal realistically with the FM overshoot problem, it is recognized that amplitude overshoot doesn't occur only in the input audio low-pass filter circuit but rather is an anomaly that is distributed throughout the FM stereo generator/exciter system. To provide a universal solution to the problem the cure must be made external to the FM exciter/stereo generator. This will interest a cost-conscious FM station manager since this does not involve scrapping a presently working and reliable FM exciter/stereo generator system.